This invention relates to a process for preparing insulating substrates, more specifically electrically insulating substrates suitable for preparing printed circuits by electroless deposition techniques.
It is widely known to utilize substrates comprised of a plurality of fibrous sheets or webs impregnated with thermosettable resin, most usually epoxy resin, as a base on which to form a firmly adherent metal layer or pattern by electroless deposition in order to form printed circuit boards. In the art, prior to electroless deposition or preparatory steps therefor (e.g., etching, seeding or the like), the substrate is subjected to heat and pressure conditions to cure the thermosettable resin and thus form a cured integral laminate onto and/or in which printed circuit patterns are to be formed.
Hereinafter, the above-described type of thermosettable resin impreganted fibrous material, prior to the curing step, will be referred to as an "insulating substrate." After the curing step, the material is referred to as a "cured insulating substrate."
In the prior art, difficulties have been encountered in forming a strong bond between the surface of the cured insulating substrate and an electroless deposited metal layer. Peel strength on the order of 8-10 pounds/square inch or higher is desired but difficult to achieve. One approach in the prior art to improve the bonding strength between a surface of a cured insulating material and an electrolessly deposited metal thereon has been the application of an intermediate adhesive layer prior to electroless metal coating or precursor steps thereof. The prior art describes the application of the adhesive in at least a partially cured state from a transfer base material to the surface of the insulating substrate and thereafter removing the transfer base material and laminating the coated insulating substrate in a conventional manner to form a cured insulating substrate. If the adhesive was not completely cured prior to the transfer operation, curing is completed under the heat and pressure conditions of lamination, say 1,000-1,500 psi at 340.degree. F. for 45 minutes for phenolic impregnated substrate and 200-275 psi at 340.degree. F. for 1/4-1/2 hour for epoxy impregnated substrate.
U.S. Pat. No. 3,956,041 by Polichette et al. discloses a transfer sheet process where a metal foil or plastic transfer sheet is coated with an adhesive composition whch is partially hardened to the "B" stage to produce a solvent-free, non-tacky, not completely hardened surface. Adhesives comprising nitrile rubber/thermosetting phenolic resin are contemplated. The adhesive surface of the transfer material is brought into contact with the surface of an insulating substrate. After subjecting the laminate to the conventional heat and pressure conditions, the transfer base material is removed, e.g., by peeling and then electroless metal plating is carried out.
U.S. Pat. Nos. 3,925,138 and 4,001,460 by Shaul et al. disclose processes which in certain embodiments appear to be similar to Polichette et al., except that the adhesive is substantially fully cured on the transfer base material prior to lamination.
The transfer base material of the prior art can be paper, plastic sheeting, metal foil and the like. Most usually, the transfer base material is selected so that it can be peeled off of the laminate after consolidation by heat and pressure. However, with the use of metal foil transfer base materials, a release agent of some type is preferred to aid peeling, or the metal foil could be etched (dissolved) away after consolidation. See the Shaul et al. patents and U.S. Pat. No. 3,948,701 to Fasbender et al. Also, see the Fasbender et al. patent concerning at least some degree of preliminary hardening, i.e., preliminary condensation in the case of hardening by the condensation mechanism, of the adhesive on the transfer base or carrier material prior to lamination with the insulating substrate.
MacDermid, Inc. of Waterbury, Conn., markets products under the trade name Pladd II.RTM.. One of these products is believed to be a metal foil carrying a cured resin. The product is designed for application to a substrate for printed circuit boards. The foil is to be etched away after a laminating cycle.